Generally, satellite positioning systems (SPS), (for example, Navstar or Global Positioning System (GPS) is a widely used SPS system), provide an invaluable service that has perhaps exceeded the imagination of the designers of the systems. For example, SPS systems are used in military applications (such as providing targeting information for smart bombs, navigation guidance for vehicles and foot soldiers, etc.), commercial applications (such as tracking delivery trucks, measuring the performance of vehicles, providing position location to a person on foot or in a vehicle, etc.), and medical and scientific applications (such as assisting in the location of persons in need of assistance, tracking animal migrations, etc.).
While SPS systems have become widely used today, there remains a major problem that hinders their use in certain situations. Since the satellites are in high-earth orbit (for example, the satellites in the GPS system orbit the Earth at approximately 11,000 nautical miles), the signals that are transmitted by the satellites are usually very weak by the time they reach a SPS receiver. Because the satellites must provide their own power (typically through the use of solar panels), the transmit signal strength cannot simply be increased, since doing so may consume more power than the solar panels are able to provide. Due to the relatively low transmit signal power, SPS signals are attenuated by thick foliage, buildings, tunnels, etc. to a point where the SPS signals fall below a minimum signal power threshold and becomes difficult (if not impossible) for the SPS receiver to detect.
When a SPS receiver is first turned on, it must be able to receive the transmitted signals from a certain number of SPS satellites (for example, in the GPS system, the GPS receiver should be able to receive the transmitted signal from at least three or four GPS satellites) before it can determine its position and the current time. Each satellite in the SPS system transmits a unique signal that can be used by the SPS receiver (in conjunction with signals from other SPS satellites) to calculate the SPS receiver's position and time. One of the most vital pieces of information that is transmitted in the SPS signal is a highly accurate timing signal. Along with the timing signal, other information (commonly referred to as ephemeris data) is transmitted by a SPS satellite. The timing signals transmitted by the SPS satellites are usually referenced to a particular time, for example, Greenwich Mean Time. The differences between the various timing signals received by the SPS receiver and its own internal clock are then used to calculate the position of the SPS receiver and the current time.
The low transmit signal power, when compounded with the natural attenuation (signal strength being inversely proportional to distance) of the transmit signals, may prevent SPS receivers from being able to acquire the signal that is transmitted from the SPS satellites (or from a sufficient number of SPS satellites). A low transmit signal power that is further attenuated by objects in the operating environment of a SPS receiver can slow down the acquisition process to the order of several minutes (or more) or it may prevent the acquisition from occurring altogether.
It has been noted that if a SPS receiver can be provided with an accurate timing reference (along with ephemeris data) through a source other than the actual signals transmitted by the SPS satellites, the acquisition process can be accelerated (or completed if it was previously unable to do so). Moreover, the more accurate the timing reference that is provided through an alternate source, the easier the acquisition process becomes.
A widely available communications network that can itself be used to provide an accurate timing reference (along with ephemeris data) is the cellular telephone network. Certain types of cellular telephone networks have highly accurate built-in clocks. For cellular telephone networks without built-in clocks, such clocks may be added at the cell-sites (base stations). One proposed solution for helping the SPS receiver achieve faster satellite acquisition makes use of an enhanced SPS receiver with a built-in cellular telephone network interface. The enhanced SPS receiver can then communicate with the cellular telephone network and obtain an accurate timing reference from a clock built into the cellular telephone network.
One disadvantage of the prior art is that although there exists a large networks of cellular telephone systems, only a relatively small number of them are equipped to provide the accurate timing reference. For the majority of the remaining cellular telephone systems, an accurate timing reference needs to be installed at high cost.
A second disadvantage of the prior art is that while large networks of cellular telephone systems exist, there are large parts of the world (rural and under-developed regions) where there is no cellular telephone coverage. Therefore, in areas with no cellular telephone coverage, the SPS receivers may still trouble acquiring the signals transmitted by the SPS satellites.